Comparative mutational analysis of

Human immunodeficiency virus type 1 (HIV-1) and human T cell
leukemia virus type II (HTLV-2) use a similar mechanism for –1
translational frameshifting to overcome the termination codon in
viral RNA at the end of the gag gene. Previous
studies have identified two important RNA signals for frameshifting,
the slippery sequence and a downstream stem–loop structure.
However, there have been somewhat conflicting reports concerning
the individual contributions of these sequences. In this study we
have performed a comprehensive mutational analysis of the cis-acting
RNA sequences involved in HIV-1 gag–pol and HTLV-2 gag–pro frameshifting. Using an in vitro translation
system we determined frameshifting efficiencies for shuffled HIV-1/HTLV-2
RNA elements in a background of HIV-1 or HTLV-2 sequences. We show
that the ability of the slippery sequence and stem–loop
to promote ribosomal frameshifting is influenced by the flanking
upstream sequence and the nucleotides in the spacer element. A wide
range of frameshift efficiency rates was observed for both viruses
when shuffling single sequence elements. The results for HIV-1/HTLV-2
chimeric constructs represent strong evidence supporting the notion
that the viral wild-type sequences are not designed for maximal
frameshifting activity but are optimized to a level suited to efficient
viral replication.